This invention relates to transmitters. More specifically, it relates to transmitting apparatus and methods using non-linear amplifiers to produce linear amplification of an input signal, particularly one with high peak-to-average power, with an improved efficiency.
Modern wireless communication systems have a need for transmitters with an improved efficiency while avoiding distortion. The greatest operating cost of a base station is the cost of the electrical energy consumed by the amplifiers of the transmitter. Any significant improvement in efficiency over the known systems presents a like significant advantage in energy and cost savings.
Use of a single power amplifier, e.g. class A, limits the efficiency of the transmitter to the inherent efficiency of the amplifier, and there is no way to improve the efficiency. Various modulation schemes and communication system architectures have been proposed to improve efficiency.
One known amplifier architecture is the Chiriex system using passive stubs to control distortion. Icefyre Semiconductor has recently reported a Chiriex type amplifier system using two amplifiers plus a combiner. A digital modulation system is also known, but it requires a back off of about 5 dB PA (Power Amplifier), which introduces distortion. Ideally, however, it is optimal to operate with high efficiency at peak power.
As another example of known transmitters, D.C. Cox proposed in 1974 using two amplifiers in a “LINC” (linear amplifier with non-linear components) structure to overcome this limit on efficiency, while retaining linearity.
The LINC architecture uses a signal separator to convert the amplitude (or phase) modulation of an input signal S(t) into phase modulation of two constant (uniform amplitude) envelope signals, S1(t) and S2(t). Non-linear amplifiers, each operating at peak power with high efficiency, receive and amplify each of the constant envelope signals S1(t), S2(t) transmitted along two parallel branches. When summed at a passive, hybrid combiner, the signal envelopes S1(t) and S2(t) reproduce a linear amplification of the original signal. In this manner, the RF microwave power amplifiers can be operated at saturation with two constant envelope signals yielding maximum amplifier efficiency and, in principle, perfect linearity.
The known LINC constant envelopes signals S1(t) and S2(t) can be represented in a signal component vector or phasor diagram, such as shown in FIG. 12, where the I axis represents the in-phase component and the Q axis represents the quadrature phase component S1 and S2 are shown as two rotating vectors S1(t) and S2(t), each with an amplitude rmax/2 and a phase angle θ(t) with respect to the signal S(t), when in complex form, S(t)=r(t)jφ(t), 0≦r(t)≦rmax.
A standard hybrid combiner, i.e., one matched at all ports with high isolation between its two branches, gives excellent linearity, but it degrades the efficiency of the LINC transmitter system. Indeed, the efficiency of a conventional LINC transmitter decreases when the peak-to-average power of the output signal increases. Yet most modulation techniques in wireless communication systems today, e.g., CDMA, WCDMA, MQAM (M 64), and OFDM signals, present a high peak-to-average power. With such signals applied to known LINC systems, the combining structure degrades the efficiency. However, the LINC structure nevertheless is more efficient than the use of only one branch with a single RF class A amplifier. For wireless communication systems, there is therefore a need to improve the efficiency of LINC systems particularly for use with the input signals having a high peak-to-average power profile.
It is therefore a principal object of this invention to provide a transmitter system that operates with a substantially improved efficiency over known LINC systems, particularly when the input signal has a high peak-to-average power.
Another object is to provide this improved transmitter efficiency while maintaining a substantially linear output signal.
A further object is to provide this improved efficiency and good linearity using standard components with attendant favorable component cost and availability.
Still another object of the invention is to provide a transmitter system with the foregoing advantages that can operate over a wide range of frequencies, including RF and microwave.
Yet another object of the invention is to provide a transmitter system with the foregoing advantages that can assume a variety of configurations each of which can be optimized for a specific application.